Intra-resonator frequency multiplication of solid-state or optically pumped semiconductor (OPS) radiation laser radiation in optically nonlinear crystals is commonly used to generate ultraviolet UV laser radiation. The frequency multiplication may be carried out within an active resonator in which the fundamental laser radiation is being generated or may be carried out separately in a passive traveling-wave ring resonator that provides enhancement of the frequency conversion by re-circulating the radiation being converted through the optically linear crystal such that radiation not converted to UV radiation generated after one pass can generate further UV radiation on a subsequent pass.
UV radiation must be separated from radiation being converted to provide UV radiation output of the resonator. This is usually effected by including a mirror including a dichroic coating in the laser resonator. Such a mirror may be one of the mirrors defining the resonator, i.e., either a terminating mirror or a mirror folding the resonator axis, in which case the dichroic coating would be arranged to transmit the UV radiation and reflect the radiation being converted. The mirror may also be a separate mirror with the dichroic coating arranged to transmit the radiation, usually visible radiation, being converted, and to reflect the UV radiation out of the resonator, transverse to the resonator axis.
Problems are often encountered with dichroic-coated elements as such elements cannot be made entirely loss free, and are typically more lossy the shorter the wavelength of the radiation. Further, optically nonlinear crystal materials used for converting radiation to UV wavelengths are subject to degradation by the UV radiation being generated. Such optically nonlinear crystal materials include, but are not limited to, β-barium borate (BBO) and cesium lithium borate (CLBO). These problems must be taken into account when deciding how to best accomplish the UV output separation. As UV degradation can not be entirely avoided, most commercial lasers in which such optically nonlinear crystals are used for UV generation usually include an arrangement for periodically moving the crystal as degradation appears on parts of the crystal through which the UV radiation passes.
FIG. 1 schematically illustrates one prior-art arrangement 10 that is used to generate UV radiation having a wavelength of 266 nanometers (nm) by frequency-doubling radiation having a wavelength of 532 nm. In arrangement 10 an optically nonlinear crystal 12, has an entrance face 12 and an exit face 14. The crystal, here, is assumed to be a BBO crystal. The crystal is cut such that 532 nm radiation incident on face 12 at about the Brewster angle θB (for that wavelength) travels along a longitudinal axis 16 of the crystal and exits face 14 also at the Brewster angle, i.e., faces 14 and 16 are parallel to each other. The 532 nm radiation is plane-polarized with the polarization plane being parallel to the plane of incidence of face 14 (p-polarized), as indicated by arrow P1. 266 nm radiation generated in the crystal follows the same path as the 532 nm radiation, and exits face 14 of the crystal at the Brewster angle for the 266 nm wavelength. There is a difference of approximately 1° between the exit angles of the 532 nm and 266 nm radiation. The 266 nm radiation is plane-polarized with the polarization plane being perpendicular to the plane of incidence of face 14.
The 532 nm radiation and 266 nm radiation are incident at the Brewster Angle for 532 nm radiation on a beamsplitter 18 having front and rear surfaces 20 and 22 respectively. There is a dichroic coating 24 on front surface 20. Rear surface 22 is uncoated. The dichroic coating reflects more than 95% of the incident 266 nm radiation and transmits more than 95% of the 532 nm radiation. By way of example, with commercially available coatings, the transmission of 532 nm radiation may be as high as 99.7% and the reflection of the 266 nm radiation may be as high as 96%. There is essentially no reflection of 532 nm radiation from surface 22 of the beamsplitter because of the Brewster-angle incidence of the radiation at the surface. Those skilled in the art will recognize, without further illustration or detailed description, that the path of the 532 nm-radiation depicted in FIG. 1 would be collinear with the longitudinal axis of a resonator in which the crystal was located.
Exit surface 16 of crystal 12 creates about 20% loss of the 266 nm radiation. This is because the 266 nm radiation is polarized in a plane perpendicular to the plane of incidence of surface 16 (s-polarized) as indicated by arrowhead P2. It is possible, in theory at least, to reduce this loss by adding a suitable antireflection coating to surface 16. It has been found, however, that in a passive ring-resonator, such a coating rapidly fails. It is believed that this failure is due to local heating in the coating by the 532 nm radiation.
Preferably coating 24 is deposited by a Q-Plate™ process. This process is an ion-assisted deposition process capable of producing coatings with very low surface roughness, for example, on the order of about 1.4 Ångstrom units (Å.U). Such coatings are available from Coherent, Inc., of Santa Clara, Calif., the assignee of the present invention. Dichroic coating 24, deposited by the Q-Plate™ process, has exhibited a long lifetime in a passive ring resonator. Eventual UV degradation is, however, inevitable. This long lifetime, was also observed in a similar arrangement wherein 488 nm radiation is frequency-doubled to provide 244 nm UV radiation. The coating is highly efficient at extracting the 266 nm UV power with very loss of the 532 nm radiation. There is a need, however, for a UV out-coupling arrangement for a frequency-doubling resonator that does not exhibit the UV loss of the arrangement of FIG. 1 and does not require an optically nonlinear crystal having antireflection coatings in the path of radiation circulating in the resonator. Preferably, the arrangement should allow for translation of the crystal when parts of the crystal surfaces are degraded by UV radiation.